In recent years, cost-cutting measures for the manufacturing cost of cans have been taken to expand the demand for steel cans. An example of the cost-cutting measures for the manufacturing cost of cans is a reduction in raw-material cost. Progress has been made in reducing the thicknesses of steel sheets used for both two-piece cans, which are formed by drawing, and three-piece cans, which are mainly formed by cylinder forming. However, a simple reduction in the thickness of a conventional steel sheet reduces the strength of a can body. Thus, high-strength thin steel sheet for a can is desired for these uses.
As a method for manufacturing high-strength steel sheet for a can, JP 5-195073 discloses a method including subjecting a steel containing 0.07%-0.20% C, 0.50%-1.50% Mn, 0.025% or less S, 0.002%-0.100% Al, and 0.012% or less N to rolling, continuous annealing, and skin pass rolling to afford a steel sheet having a proof stress of 56 kgf/mm2 or more.
JP 59-50125 discloses a method including subjecting a steel containing 0.13% or less C, 0.70% or less Mn, 0.050% or less S, and 0.015% or less N to rolling and continuous annealing and that a steel sheet has a yield stress of about 65 kgf/mm2 after lacquer baking in an Example.
JP 62-30848 discloses a method including subjecting a steel containing 0.03%-0.10% C, 0.15%-0.50% Mn, 0.02% or less S, 0.065% Al, and 0.004%-0.010% N to rolling, continuous annealing, and skin pass rolling to afford a steel sheet having a yield stress of 500±50 N/mm2.
JP 2000-26921 discloses a method including subjecting a steel containing 0.1% or less C and 0.001%-0.015% N to rolling, continuous annealing, overaging, and skin pass rolling to afford a steel sheet having a temper designation of up to T6 (a hardness of about 70 (HR30T)).
Nowadays, a steel sheet having a yield strength of about 420 MPa is used for bodies of three-piece cans. The steel sheet is required to have a thickness reduced by several percent. It is necessary to have a yield strength of 450 MPa or more to meet the requirement and maintain the strength of can bodies.
When a steel having high C and N contents is produced and formed into a slab, cracking can occur at a corner (hereinafter, referred to as a “slab corner”) of a long side and a short side of the cross section of the slab in a continuous casting process. In the case of a vertical-bending type or bow type continuous casting machine, the slab undergoes bending deformation or unbending deformation (only in the vertical-bending type continuous casting machine) at high temperatures. Such a steel with high C and N contents has poor high temperature ductility, thus causing cracking during deformation. When the slab corner is cracked, it is necessary to perform, for example, surface grinding. This disadvantageously causes a reduction in yield and an increase in cost.
In the present circumstances, the high-strength steel sheets described in the related art have high proportions of C and N, which function as solid-solution strengthening elements, and thus are highly likely to be cracked at slab corners in a continuous casting process.
It could therefore be helpful to provide a steel sheet for a can, the steel sheet having a yield strength of 450 MPa or more and being free from cracking at a slab corner in a continuous casting process, and a method of manufacturing the steel sheet for a can.